20 AGRICULTURAL EXPERIMENT STATION. 



is really carnivorous in type. At the outside of the brushes are 

 the remaining- maxillary structures, usually lumped under the 

 term palpi. In the middle of the under side is a triangular or 

 quadrate brown plate, the mentum, which represents the labium 

 or lower lip. The edges of this mentum are toothed or serrated 

 and the variations assist in the determination of the species. 

 The variations in these mouth structures are shown on the plates 

 illustrating the different larvae. They cannot be seen except with 

 a compound microscope after proper dissection and they are 

 not essential to a i-ecognition of the larva. 



The thoracic segments are so massed together that they are 

 not easily distinguished except at the sides, where tubercles, 

 giving rise to hair tufts, mark their edges. Except for its gen- 

 eral shape and proportion the thorax offers nothing of import- 

 ance for ready classification. 



Then follows a series of seven segments, which usually re- 

 semble each other closely and which differ little in the species 

 except for the leng-th of the hair tuftings which come from the 

 sides. On the eighth segment there is on each side a patch of 

 scales with a definite arrangement, the scales all very much alike, 

 3^et not alike for any two species. The character of this patch 

 of scales or comb and the shape of the individual scales is of the 

 greatest importance in classification and some closely allied 

 species are distinguished with certainty only by a reference to it. 

 The arrangement of the scales and their form is shown for each 

 species. 



The anal tube or siphon is also a, 'process of the eighth seg- 

 ment, and this varies in length and in form ; there is more varia- 

 tion here, within specific limits, than in some other structures, 

 and yet there is a characteristic appearance in some cases that, 

 in connection with the antenna, decides that species at once. On 

 the under surface of this siphon is a double row of spines ex- 

 tending from base to or not much beyond the middle and these 

 spines are also characteristic in shape and reasonably constant 

 in number. The term pecten is applied to these series of spines 

 and that term in the diagram is erroneously referred to the scale 

 patch. 



The function of this tube is to enable the larva to reach 

 through the surface of the water to the outer air to get a supply 

 of oxygen, and for most of the species this is the only way in 

 which it can be obtained. Cut off access to the air and the larva 

 drowns, like any other air-breathing animal. By putting oil on 

 the surface of a pool inhabited by mosquito larvae, access to the 

 air is shut off and they die. 



